1. Field of the Disclosure
This invention pertains to a method and an apparatus for preparing a relief printing form. More specifically, this invention describes a method and an apparatus for preparing the relief printing form from a photosensitive element that is exposed to actinic radiation in an environment having controlled oxygen concentration.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials including corrugated carton boxes, cardboard boxes, continuous web of paper, and continuous web of plastic films. Flexographic printing plates are a form of relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element.
Flexographic printing forms are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Typically, the printing form precursor is uniformly exposed through its backside, i.e., backflashed, to a specified amount of actinic radiation to form a floor, and is imagewise exposed through its front side with the same actinic radiation that was used for the backflash exposure. The imagewise exposure can be through an image-bearing art-work or a phototool, such as a photographic negative or transparency (e.g. silver halide film), that is held in intimate contact under vacuum to the photopolymerizable layer, so called analog workflow. Alternatively, imagewise exposure can be through an in-situ mask having radiation opaque areas and transparent areas that had been previously formed above the photopolymerizable layer, so called digital workflow. The precursor is exposed to actinic radiation, such as ultraviolet (UV) radiation, to selectively cure the photopolymerizable layer. The actinic radiation enters the photosensitive element through the transparent areas and is blocked from entering the photopolymerizable layer by the black or opaque areas of the transparency or in-situ mask. The areas of the photopolymerizable layer that are exposed to the actinic radiation cure or hardened and crosslink. The unexposed areas of the photopolymerizable layer that were under the opaque regions of the transparency or in-situ mask during exposure do not crosslink or cure (i.e., harden). The uncured regions are soluble to solvents used during washout development and/or can melt, soften, or flow upon heating. The plate is then subjected to a processing step wherein the unexposed areas (i.e., uncured areas) are removed by treating with a washout solution or heat leaving a relief image suitable for printing. If treated with washout solutions, the plate is subsequently dried to remove solvents that may be absorbed by the plate. The printing plate can be further exposed to UV radiation to ensure complete polymerization and to remove surface tackiness. After all desired processing steps, the plate is then mounted on printing press to print the formed relief image onto a substrate.
Analog workflow requires the preparation of the phototool, which is a complicated, costly and time-consuming process requiring separate processing equipment and chemical development solutions. In addition, the phototool may change slightly in dimension due to changes in temperature and humidity. The same phototool, when used at different times or in different environments, may give different results. Since a phototool is created for each printing plate according to the color of ink being printed in a multicolor image, dimensional instability of the phototool can result in the mis-registration of multicolor images during printing. Use of a phototool also requires special care and handling when fabricating flexographic printing forms to ensure intimate contact is maintained between the phototool and plate. In particular, care is required in the placement of both the phototool and the plate in the exposure apparatus along with special materials to minimize air entrapment during creation of a vacuum to ensure intimate contact. Additionally care must be taken to ensure all surfaces of the photopolymer plate and phototool are clean and free of dust and dirt. Presence of such foreign matter can cause lack of intimate contact between the phototool and plate as well as image artifacts.
An alternative to analog workflow is termed digital workflow, which does not require the preparation of a separate phototool. Photosensitive elements suitable for use as the precursor capable of forming the in-situ mask in digital workflow are described in U.S. Pat. No. 5,262,275; U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,238,837; U.S. Pat. No. 6,558,876; U.S. Pat. No. 6,929,898; U.S. Pat. No. 6,673,509; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,037,102; and U.S. Pat. No. 6,284,431. The precursor or an assemblage with the precursor includes a layer sensitive to infrared radiation and opaque to actinic radiation. The infrared-sensitive layer is imagewise exposed with laser radiation whereby the infrared-sensitive material is removed from, or transferred onto/from a superposed film of the assemblage, to form the in-situ mask having radiation opaque areas and clear areas adjacent the photopolymerizable layer. The precursor is exposed through the in-situ mask to actinic radiation in the presence of atmospheric oxygen (since no vacuum is needed). Furthermore, due in part to the presence of atmospheric oxygen during main exposure the flexographic printing form has a relief structure that is different from the relief structure formed in analog workflow (based upon the same size mask openings in both workflows). Digital workflow results in the relief image having a different structure of the raised surface areas. In particular, the fine raised surface of dots (i.e., the individual elements of a halftone image) is typically smaller, with a rounded top, and a curved sidewall profile, which is often referred to as dot sharpening effect. Dots produced by analog workflow are typically conical and have a flat-top. The relief structure formed by digital workflow results in positive printing properties such as, finer printed highlight dots fading into white, increased range of printable tones, and sharp linework. As such, the digital workflow because of its ease of use and desirable print performance has gained wide acceptance as a desired method by which to produce the flexographic printing form.
It is known by those skilled in the art that the presence of oxygen (O2) during exposure in free-radical photopolymerization processes will induce a side reaction in which the free radical molecules react with the oxygen, while the primary reaction between reactive monomer molecules occurs. This side reaction is known as inhibition (i.e., oxygen inhibition) as it slows down the polymerization or formation of crosslinked molecules. Many prior disclosures acknowledge that it is desirable for photopolymerization exposure to actinic radiation to occur in air (as is the case for digital workflow), under vacuum (as is the case for analog workflow), or in an inert environment. Oftentimes, nitrogen is mentioned as a suitable inert gas for the inert environment. The implication is that the nitrogen environment is one that contains substantially less than atmospheric oxygen to the extent that all oxygen is removed, or something less than about 10 ppm of oxygen. Nitrogen with oxygen impurity concentration level less than 10 ppm is readily commercially available.
A commercially important substrate for packaging graphics printing is corrugated paperboard. Corrugated paperboard includes a corrugating medium, which is a layer of pleated or multi-grooved paperboard typically called flute, adjacent a flat paper or paper-like layer referred to as liner. A typical embodiment of corrugated paperboard includes the flute layer sandwiched between two liner layers. Other embodiments of corrugated paperboard can include multiple layers of flute and liner. The fluted interlayer provides structural rigidity to the corrugated board. Since corrugated paperboard is used as packaging and formed into boxes and containers, the liner layer forming an exterior surface of the corrugated paperboard is frequently printed with the necessary identifying information for the package. The exterior liner layer often has slight indentations due to the uneven support of the underlying flute layer. A problem often encountered with printing onto corrugated board substrates is the occurrence of a printing effect that is typically referred to as fluting or banding, and can also be called striping or washboarding. Fluting typically occurs when post printing, that is printing the liner on the exterior surface of the corrugated paperboard, after the corrugated paperboard has been assembled. The fluting effect manifests as regions of dark printing, i.e., bands of higher density, alternating with regions of light printing, i.e., bands of lower (or less than high) density, that correspond to the underlying fluting structure of the corrugated board. The darker printing occurs where uppermost portions of the pleated interlayer structure support the printing surface of the liner. The fluting effect can be apparent in areas of a printed image having tones or tint values where the inked areas represent a fraction of the total area, as well as, in areas of the printed image where the ink coverage is complete or a solid. However, this fluting effect is found to be more pronounced when printing with a relief printing form that was produced using the digital workflow.
So a need arises for a modified process and apparatus for preparing relief printing forms from a photosensitive element (precursor) that is easy to use which provides printing forms having a relief structure similar to or better than the printing forms of the analog workflow process, and mitigates the disadvantages of the conventional digital workflow process, particularly, those resulting from the presence of oxygen during exposure to actinic radiation.